The present invention relates generally to input/output circuits and more particularly to low-skew input/output level-shift circuits for integrated circuits.
Integrated circuit data rates and associated clock frequencies have been dramatically increasing the past few years, and the rate of this increase shows no signs of abating. Fortunately, circuit designers have developed various techniques for keeping pace. But these techniques place severe demands on the integrity of a device's data and clock signals.
For example, at double-data rate memory interfaces, data is transferred at each edge of a clock signal. To optimize data transfers, clock edges should have a very low level of skew between their rising and falling edges, since a high level of skew can impair device performance. As a comparison, when data is transferred at only one clock edge, rising and falling edge skew is more forgivable so long as the overall clock cycle period remains stable.
Similarly, it is desirable that rising and falling edges of data signals have low skew. Particularly in systems where data is to be recovered by a second integrated circuit, it is important that a first integrated circuit provide a data output having an open “eye” such that data can be accurately retimed.
But data and clock signals are typically handled on integrated circuits by inverters or similar logic gates. These circuits are inherently single-ended in nature, that is, they provide single-ended signals that are more prone to skew than differential signals. At its simplest, an inverter receiving a single-ended signal switches its output when an input signal crosses a threshold voltage. But skew results if the threshold does not match the cross point of the rising or falling edges of the input signal. The result is that the delay through an inverter for a rising edge may differ from the delay for a falling edge. This is particularly true when an input signal and a receiving inverter have different voltage ranges. For a clock signal, this can make double-data rate clocking more difficult. For data signals, this can close the “eye” needed for data recovery.
Thus, what is needed are circuits, methods, and apparatus for providing low-skew input and output level-shift circuits.